Q. "My child is 15 and seems to have no motivation.

"Other boys his age get summer jobs and he doesn't even try. He reads Westerns and watches TV (the soap operas!) or "hangs out," as he calls it. He's not very interested in sports or school or even girls -- only cars.

"Driving is the big thing in his crowd, but when one of the 16-year-olds gets the family car, they just drive aimlessly about from a shopping center to a pizza parlor. And then they complain that there is nothing to do.

"Why is it that some kids hang around and others stay so busy?"

A. Motivation is built on success and success is the source of a child's self-esteem. It takes self-esteem for a child to tackle the world.

The goals vary with the age of the child.

To feel successful, a pre-schooler needs to move his body well enough to make his arms and legs obey him more surely every year. A grade-school child refines these skills and combines endurance with them, spending hours at a time skating, jumping rope, playing ball. He also must make his fingers draw with increasing precision and organize his mind enough to read and write and compute as well as anyone else. The better he performs a skill, the more likely he is to improve it and learn new ones because he will have the faith in himself to try.

A teen-ager has a different yardstick of competence. He judges himself on his ability to survive on his own.

Unless he thinks he can, he will "hang out" (not because he is unmotivated, but because he is afraid he'll fail). And he will dream of a car -- not to be sporty or sophisticated -- but to rescue him, because he thinks he can't rescue himself.

Although a 15-year-old may say there is nothing to do, he scarely has time to learn all the skills he needs before he is grown.

By the time your child finishes high school, he -- and certainly she -- needs to know how to type well enough to write a 5-page paper without staying up all night; use the Scorpio system in the library; save money for a major goal; keep a checkbook in balance; drive a car and change its oil, tires and carburetor filter; mend well enough to take up a hem, sew buttons and make curtains for a college apartment; iron a shirt or anything else; know what makes a meal nutritious and cook it with reasonable ease; shop for the ingredients without spending too much (or tumbling for the television commercials); clean a house in a day; mow a lawn; change, bathe and entertain a baby, and get anywhere in a city on public transportation.

A child doesn't learn these jobs with one or two lessons, but with practice and by being depended upon to do his share. And yes, he will complain, but when parents expect a child to be responsible, they give him a real measure of respect.

When he knows he can do the work, he'll have the nerve to look for a job as a baby-sitter, a bus boy, a housecleaner, an au pair ; a gardener. Money is a great motivator at any age; but especially now.

The weekend skills -- the ones you use to keep your house in shape -- are the ones that have the greatest value, however, both in wages and in self-esteem.

That's why your teen-ager should be learning how to replace a washer on a faucet, rewire a lamp, plaster small holes in the wall, patch a roof and prune a tree. And he should do these jobs so often and be expected to do them so well he will be able to charge for his work -- but not, of course, at home. A family is not a business.

A certain amount of hanging out is good for the soul, but the teen-ager who knows he can survive will feel too successful and motivated -- and busy -- to do it very often.